It is sometimes necessary or desirable to sense one or more of a variety of compositional blood parameters. Examples of such compositional blood parameters include the presence and/or concentration of blood constituents, such as blood gases, hydrogen ions (pH), other electrolytes, glucose, red blood cells and the like. Blood parameters can be sensed and appropriate measurements taken in real time, and this may be accomplished in vivo as set forth for example in Maxwell U.S. Pat. No. 4,830,013, in vitro or in an extracorporeal blood loop as disclosed in Cooper U.S. Pat. No. 4,640,820.
Many blood parameters of interest can advantageously be sensed optically and this can be accomplished, for example, using fluorescent, absorption or transmittance techniques. Optical sensing typically requires a disposable apparatus and a reusable instrument. The disposable apparatus includes an optical sensor for sensing the blood parameter of interest. The optical sensor is responsive to an optical input signal to provide an optical output signal related to the blood parameter of interest.
In order to provide optical coupling between the disposable apparatus and the reusable instrument, it is necessary to optically connect the instrument to the disposable apparatus. This may be accomplished, for example, by coupling an instrument head to the disposable apparatus. It is important that such optical connection be made with minimal, known optical losses and be repeatable when the disposable apparatus is removed and replaced again on the instrument head.
In the past, the instrument head has been pressed tightly against a surface of the disposable apparatus which contains a shallow, open-ended recess. The instrument head contains an optical transmission member in the form of an optical fiber bundle which is then flush with the outer end of this recess. With this construction, the shallow recess in the disposable element defines an air gap which provides a known and reproducible optical throughput.
This construction works very well so long as there is no moisture in the recess. Unfortunately, however, there is a danger that moisture will accumulate in the recess, and if this occurs, the optical coupling loss changes, the blood parameter readings are upset and repeatability is lost. For example, moisture may form in the recess as the result of condensation of vapors during storage of the disposable apparatus or during setup procedures in the operating room. Moisture may get in the recess when the disposable apparatus is removed after calibration and reattached to the instrument head. Also, condensation from cold lines and cold equipment in the operating room can introduce moisture into the recess during use of the disposable apparatus.